The invention relates generally to a communication system for performing communications between communication terminals on the basis of a communication protocol such as SIP (Session Initiation Protocol), and more particularly to a communication system and a communication terminal each enabling other communication terminal to make a response to an incoming call as a substitute for a communication terminal receiving the incoming call according to SIP.
There has hitherto been proposed a communication system (refer to, e.g., Non-Patent document 1) enabling other communication terminal to make a response to an incoming call as a substitute for a communication terminal receiving the incoming call according to the communication protocol such as SIP (Session Initiation Protocol). This communication system is configured as, for example, as shown in FIG. 1, a communication system utilizing so-called IP telephones. To be specific, IP telephones (which will hereinafter be called communication terminals) 11, 12 are connected to a LAN 15 (an IP network), and the LAN 15 is connected to an IP network 30 (e.g., the Internet) extending in a wide area via a communication control device 14 (e.g., a router, etc.). Then, other communication terminal 20 connected to the IP network 30 can perform voice communications (RTP: Real-Time Transport Protocol) with any one of the communication terminals 11, 12 connected to the LAN 15 via the IP network 30, the communication control device 14 and the LAN 15.
Each terminal has a functional configuration as shown in, for example, FIG. 2.
In FIG. 2, each of the communication terminals 11, 12 has a layered configuration built up by a user interface unit 101, a line control unit 102, a transaction unit 103 and a transport unit 104. The transport unit 104 has a function of sending a message via the LAN 30, and the transaction unit 103 has a function of accepting further via the transport unit 104 the message sent through the LAN 30, a function of retransmitting the message, etc. The line control unit 102 has a function of executing call control, and the user interface unit 101 has functions of generating a control message based on a user's operation, effecting output control for the user on the basis of the received message, and so on.
In this type of communication system, in a case (a case of substitute response) where the communication terminal 12 makes a response to an incoming call given from the communication terminal 20 to the communication terminal 11, for instance, the processing is executed based on a procedure shown in FIG. 3. Note that the communication terminal 20 is named [Alice], the communication terminal 11 is named [Bob], and the communication terminal 12 is named [Bill] in FIG. 3 pursuant to the description of Non-Patent document 1.
In FIG. 3, when Bob receives an incoming message INVITE F1 from Alice, Bob sends a ringing message 180 Ringing F2 back to Alice. A ringing tone is thereby outputted at Alice, and simultaneously an incoming tone is outputted at Bob. In this status, when Bill performs off-hook together with a predetermined operation (an operation for making a substitute response) in order for Bill to make a response (PICKUP) to the incoming call to Bob, a status acquisition request message SUBSCRIBER F3 is sent to Bob from Bill. Bob sends a receipt response 200 OK F4 to this status acquisition request message SUBSCRIBER F3 to Bill, and simultaneously transmits a consent message NOTIFY F5 representing consent to the status acquisition request message SUBSCRIBER F3.
Bill, when receiving the consent message NOTIFY F5, sends a receipt response 200 OK F6 to Bob and thereafter transmits a substitute incoming message INVITE Replace: Bob F7 to Alice in order to notify Alice of a purport of making a substitute response to the incoming call to Bob. Alice, when receiving the substitute incoming message INVITE Replace: Bob F7, sends a receipt response 200 OK F8 to Bill. Thereafter, Alice sends to Bob a cancel message CANCEL F9 for canceling the ringing given to Bob. Then, Bob, after transmitting a receipt response 200 OK F10 to the cancel message CANCEL to Alice, sends a message 487 F11 indicating confirmation that the incoming message INVITE F1 has been canceled. Alice transmits to Bob an acknowledgment notifying message ACK F12 with respect to the message 487 F11 sent from Bob.
Bill, when receiving from Alice the receipt response 200 OK F8 about the substitute incoming message INVITE Replace: Bob F7, sends an acknowledgment notifying message ACK F13 to Alice. Then, as described above, Bob, who has confirmed that the ringing from Alice has been canceled, sends to Bill a consent message NOTIFY F14 representing consent to Bill's making the response to the ringing from Alice, and Bill receiving this consent message NOTIFY F14 transmits to Bob a message 481 F15 indicating the confirmation thereof.
Through the procedures described above, Bob recognizes that Bill responds to the ringing from Alice, Alice recognizes that a communication with Bill is conducted by canceling the ringing to Bob, and Bill recognizes that the communication with Alice becomes possible. In this state, a call based on RTP (Real-Time Transport Protocol) is established between Alice and Bill, and voice communications can be performed between Alice and Bill. Then, when Alice sends a call disconnection message BYE F16 to Bill with an end of speech, Bill sends a receipt response 200 OK F17 back thereto, thereby terminating the communications between Alice and Bill.
According to this type of communication system, when Alice rings up Bob, Bill becomes capable of making a response to this ringing. Therefore, even if Bob (an operator of the communication terminal 11) is absent, Alice can talk with Bill (an operator of the communication terminal 12) who replaces Bob.
[Non-Patent document 1] SIPPING Working Group Internet-Draft ┌draft-ietf-sipping-service examples-05┘